3-Hydroxyacyl CoA dehydrogenases are classified as oxidoreductases. They are involved in fatty acid metabolic processes. Specifically they catalyze the third step of the beta oxidation of L-3-hydroxyacyl-CoA by NAD+. The reaction converts the hydroxyl group into a keto group: (S)-3-hydroxyacyl-CoA + NAD+ --> 3-oxoacyl-CoA + NADH + H+.